This invention relates to a demodulator circuit which receives a BPSK (Binary-Phase-Shift-Keyed) modulated carrier signal. More specifically, in the present invention the carrier signal is modulated with a particular pseudo-noise received data code. Further, the demodulator of the present invention anticipates, or has prior knowledge of, the particular pseudo-noise received data code.
The present invention is especially useful in conjunction with transponders and transponder testing. Transponders represent a class of radios which transmit a message, or data code, in response to a received signal from an inquiring device. Thus, an inquiring device, which transmits a signal to a transponder may anticipate the code which the transponder transmits.
A transponder may be useful for range measurement applications where distance is measured between the transponder and the inquiring device. In range measurement an inquiring device may be calibrated so that it "knows" when in time it will receive a transponder's transmitted code under conditions where both the transponder and inquiring device are adjacent each other in space. Then, when the transponder and inquiring device are separated in space, a delay in time of receipt of the transponder's transmitted code from the calibrated time conveys information concerning the distance between the inquiring device and the transponder. Greater distances produce greater delays.
The prior art inquiring devices in range measurement applications tend to utilize a coherent demodulator. Coherent demodulators precisely depulicate both the phase and frequency of a transmitted carrier signal in demodulating the received carrier signal. Thus, a coherent demodulator typically requires a phase locked loop type of circuit which is undesirable because it tends to be costly, complicated, and troublesome.